R.N.
I had what they diagnosed as muscle spasms a couple of weeks after my son was born. My chest would get so tight that it felt like my lungs were in a vice. I actually went to the emergency room (in an ambulance, no less), because the first time it happened I felt like I couldn't breathe and it scared my husband so badly.
I think it mostly had to do with how I was sitting when I was breastfeeding and just my body getting used to carrying around my son all the time. After a month or so it stopped happening.
Hope this helps.